The field of the present invention relates generally to data capture systems and more specifically to data readers, such as scanners and bar code reading devices.
Although the following description of this invention makes reference to bar code scanners, by way of example, the invention itself is equally applicable to other methods and systems for data reading and forms of encoded data (indicia) other than bar codes.
From an operational point of view, bar code scanners are typically operated in one of two modes, fixed or handheld. In the fixed mode of operation, objects with bar codes thereon are moved to or past a stationary bar code scanner for scanning. In the handheld mode of operation, a portable bar code scanner is typically oriented and/or moved to the bar code label to be read. For purposes of this description, the term bar code scanner shall henceforth denote a scanner of the spot scanning type, wherein an illumination spot is moved across a bar code. The bar code scanners described herein may utilize any number of scan patterns comprising any number of scan lines in any configuration suitable for bar code scanning applications and projected through any number of scan windows. Further details regarding scan lines, scan patterns, and scan windows may be found in U.S. application Ser. Nos. 60/010,935 and 08/792,829 entitled "Multi-Aperture Data Reader for Multi-Mode Operation" and Ser. Nos. 60/021,783 and 08/887,244 entitled "Bar Code Scanner with a Manually Switchable Scan Pattern" herein incorporated by reference as if fully set forth herein.
A bar code label comprises a series of parallel dark bars of varying widths with intervening light spaces, also of varying widths. The information encoded in the bar code is represented by the specific sequence of bar and space widths, the precise nature of this representation depending on the particular bar code symbology used. Methods for reading bar codes may comprise generation of an electronic signal wherein a signal voltage alternates between two preset voltage levels, one representing a dark bar and the other representing a light space. The temporal widths of these alternating pulses of high and low voltage levels correspond to the spatial widths of the bars and spaces. It is this temporal sequence of alternating voltage pulses of varying widths which is presented to an electronic decoding apparatus for decoding.
A common and well-developed method for converting the spatial bar/space sequence into a temporal high/low voltage sequence is the method of bar code reading. A bar code scanner typically has an optical system (also referred to as an opto-mechanical system) with two subsystems: an illumination subsystem which produces an illumination beam and a collection subsystem which collects and detects light. The illumination subsystem, typically comprising a light source, a focusing lens, and a scan engine, focuses an outgoing light beam to a minimum diameter, known as the waist, and generates a scan pattern so that the illumination beam, or spot, is likely to be scanned across a bar code. The collection subsystem, which typically includes a collection lens, or alternatively a concave collection mirror or functional equivalent thereof, and a photodetector, collects at least some of the light scattered and/or reflected from the bar code illuminated by the illumination beam and focuses the same onto the detector. The photodetector produces an analog signal having an amplitude determined by the intensity of the collected light. The photodetector, for example, may generate a high voltage when a large amount of light scattered from the bar code impinges on the detector, as from a light space, and likewise may produce a low voltage when a small amount of light scattered from the bar code impinges on the photodetector, as from a dark bar. When the illumination and collection paths/axes are substantially coincidental, the system is typically referred to as a retro-directive.
The illumination source in "spot" bar code scanners is typically a laser, but may comprise a coherent light source (such as a laser or laser diode) or a non-coherent light source (such as a light emitting diode). A laser illumination source offers the advantages of high intensity illumination which may allow bar codes to be read over a large range of distances from the bar code scanner and under a wide range of background illumination conditions (the area in which a bar code may be consistently read by the scanning system is commonly referred to as the depth of field). The scanner's ability to read bar codes at the outer extremes of the depth of field (far field) is, however, limited in part by collected optical power, which decreases approximately as the inverse of the square of the distance from the scanner. It is desirable for a bar code scanner to be capable of reading bar codes over an extended distance from the scanner, that is, to have a large depth of field. Many improvements have been made to bar code scanners to extend their depth of field. One such improvement is disclosed in Rudeen et al. U.S. Pat. No. 5,479,011 entitled "Variable Focus Optical System For Data Reading", the patent being hereby incorporated by reference. The Rudeen '011 patent discloses a variable width aperture disposed in the outgoing optical path thereby varying the location of the beam waist and enabling the scanner to read bar codes over a greater depth of field. Another embodiment is disclosed in Bailey et al. U.S. Pat. No. 4,978,860 entitled "Optical System for a Large Depth of Field Bar Code Scanner", the patent being hereby incorporated by reference. The Bailey '860 patent discloses a scanner utilizing a tilted detector array to extend the depth of field of the reading device. Another improvement is disclosed in Reddersen et al. U.S. Pat. No. 5,438,187 entitled "Multiple Focus Optical System for Data Reading Applications" the patent being hereby incorporated by reference. The Reddersen '187 patent discloses a system which utilizes a multiple focus lens as a means of extending the bar code scanner's depth of field.
There have been several other suggestions on how to increase the depth of field in previous bar code scanner systems. In another system, a focusing lens is designed with an axially movable lens element (such as a zoom lens) to permit changing the focusing power to change the depth of field. Such systems require complicated mechanical lens adjustment and/or may require the user to manually make focusing adjustments. It is desirable to eliminate the need for focus adjustments by the user or complicated mechanical devices.
Another previous method employed to improve depth of field for bar code scanning systems is over-filling the detector in the near field. Because collected optical power decreases approximately as the square of the distance from the scanner, many bar code scanning systems amplify the detected signal in order to read bar codes in the far field. Amplification boosts the detected signal generated by the photodetector. This amplification (or gain), however, boosts some detected signals. typically in the near field, to levels beyond the dynamic range of the bar code scanner's detection and signal processing systems. Although enabling a bar code scanner to read bar codes in the far field, amplification of detected signals generated from scanning bar codes in the near field will frequently boost the detected signal to levels outside the functioning dynamic range of the detection and signal processing systems. Increasing the dynamic range of the detection and signal processing systems (components) typically requires more expensive and complex components with a potential concomitant deterioration of bar code scanner performance. That is, a lower first pass read rate of the bar code scanner and/or a higher mis-read rate. To improve the depth of field and first pass read rate of such systems, previous collection subsystems are designed to over-fill the detector in the near field. That is, the collection lens, or alternatively the collection mirror or functional equivalents thereof, is designed so the collected light in the near field focuses a spot at the center of the detector which is larger than the detector, thereby over-filling the detector. All the collected light is not detected, thereby limiting the dynamic range of the detected and processed signal. Many other secondary factors may impact the dynamic range, but it is nonetheless desirable to limit such impact. FIG. 7 is a plot of the power collected vs. the distance from the collection lens for a certain bar code scanning system and particular conditions wherein the detector is filled when the bar code is approximately 5.3 inches from the detector. As indicated in FIG. 7, the dynamic range is limited from approximately 800 nW to 2000 nW when a bar code is scanned in a range of approximately 4-8.5 inches from the collection lens. Consequently, bar codes over a greater depth of field may be read for a given dynamic range of detection and signal processing systems.
Other previous bar code scanning systems have had optical systems which offset the collection axis from the axis of outgoing light, however, significant differences and purposes exist in these previous systems. These optical systems were designed with an offset to minimize the amount of collected light the illumination focusing lens was keeping from reaching the detector while substantially coaxially aligning the collection axis and the axis of outgoing light in retro-directive bar code scanning systems. Initially the two axes were very close together (approximately 0.1 inches apart) because the focusing lens was superimposed (off the central axis of the collection lens) in the collection lens. Moreover, the collection lens and focusing lens were aligned such that their optical axes optimally converged within the scan volume (were as close together as possible).